The November election is 137 days away. But we now have a very clear idea of what the Senate landscape -- the playing field on which the battle for control will take place -- is going to look like.

A playing field. (Photo by Marvin Joseph/The Washington Post)

Primaries have, largely, sorted themselves out in the most competitive Senate races in the country with Republicans -- so far -- avoiding the perils of 2010 and 2012 in which the party nominated a number of candidates who had major electability problems in the general election. The recently concluded Iowa primary gave Republicans their strongest nominee and the Georgia primary produced two runoff participants without the general election baggage of some of the GOP candidates in the running.

What we are left with is 12 races that can be considered truly competitive -- meaning that either one (or both) of the national parties and/or the various outside groups have or will spend money in them. The races are tipped heavily toward Democratic-held seats; 10 of the 12 contests -- including the six most vulnerable -- are currently in Democratic hands. Of the 12 states, Mitt Romney carried nine of them in 2012 -- with Michigan, Iowa and Colorado the trio that went for President Obama.

Republicans insist the playing field is actually 14 not 12 -- adding Minnesota and Oregon to the list. We remain unconvinced that Republican challengers in either of those Democratic-leaning seats have shown the ability to make the races genuinely competitive just yet. Similarly, Democratic optimism in Mississippi seem overly optimistic to us -- even if state Sen. Chris McDaniel ousts Sen. Thad Cochran in the GOP runoff next Tuesday.

Below we've ranked the 12 most competitive Senate races in the country. The number one ranked race is the most likely to switch party control.

12. Michigan (Democratic-controlled): Republican Terri Lynn Land was not the first -- or even second -- choice of many Republican strategists. But she has raised money at an impressive pace and kept this race close against Rep. Gary Peters. The question for Land is whether she can sustain it when media and voter attention ramps up in the fall. Land's campaign has protected her very carefully so far; it will be harder to do that in the stretch run with multiple daily campaign events and periodic debates. (Previous ranking: 9)

11. Georgia (Republican-controlled): We’re still awaiting the results of the Republican primary runoff on July 22 (longest runoff ever). Businessman David Perdue beat Rep. Jack Kingston 31 percent to 26 percent on primary day, but there are lots of voters up for grabs, and third-place finisher Karen Handel is backing Kingston -- thanks in no small part to Perdue’s careless decision to insult her level of schooling. We’re not sure which man gives the GOP a better shot against Democrat Michelle Nunn -- we'd lean toward Perdue -- but the Democrat is still polling well and raising big money. This remains, surprisingly, a legitimate Democratic target. (Previous ranking: 10)

10. Iowa (D): Republicans got a big break when state Sen. Joni Ernst routed the competition in the GOP Senate primary, and since then several polls have shown the race with Rep. Bruce Braley (D) to be something close to a toss-up. We still think Ernst probably got a bounce from the big primary win, but this is a swing state, and as long as she runs a credible campaign, the GOP should have a good chance in an open-seat race. (Previous ranking: N/A)

9. Colorado (D): While most people look at the North Carolina race as the one on which Senate control might swing, this race between Sen. Mark Udall (D) and Rep. Cory Gardner (R) could easily fit that bill too. Polling done by Quinnipiac in late April showed the race a dead heat and both sides acknowledge the race is and will stay close. Udall is on TV now bashing Gardner as too conservative -- particularly on abortion -- and Democrats think Gardner's record is full of bad votes. (Previous ranking: 10)

8. Alaska (D): Conservative blogger Erick Erickson this week endorsed Lt. Gov. Mead Treadwell (R), giving the long-struggling candidate a much-needed boost. But it's not going to change the fact that former attorney general Dan Sullivan is the clear frontrunner for the Republican nomination. Sullivan has both establishment (American Crossroads) and tea party (Club for Growth) money in his corner and is well on his way to a fall showdown against Sen. Mark Begich (D). (Previous ranking: 7)

7. Kentucky (R): Republicans are feeling more confident about McConnell's chances following the Republican leader's convincing primary victory last month and their sense that the GOP is quickly uniting behind him. And, President Obama didn't do Democrat Alison Lundergan Grimes any favors with his announcement on power plants earlier this month. But, as we have written in this space, McConnell's numbers suggest his vote ceiling is very low. And, in a cycle where they have very few opportunities, Democrats will pour everything they have into this one. (Previous ranking: 8)

6. Arkansas (D): Republican Rep. Tom Cotton's campaign released an internal poll claiming a lead over Sen. Mark Pryor (D). The release was meant to counteract a growing narrative that Pryor is not nearly as vulnerable as he once seemed. But the most notable part of the survey was in the trend line: Cotton was polling the race in February in 2013, when he was s till a brand new member of the House. The revelation probably won't help Cotton in his chief task right now: Humanizing himself and showing voters that he's not just a super-ambitious pol championed by national conservative groups. (Previous ranking: 5 )

5. North Carolina (D): Democrats tried to make an issue of state House Speaker Thom Tillis (R) referring to white people as the “traditional population” of North Carolina. We wouldn’t call that a campaign-stopping gaffe, but given Democrats would love to motivate minority voters in a midterm election, Tillis should probably choose his words a little more carefully. The race between Tillis and Sen. Kay Hagan (D) remains very close. (Previous ranking: 5)

4. Louisiana (D): Expect Sen. Mary Landrieu (D) to emphasize (and reemphasize) her role as head of the Senate Energy Committee for the rest of the campaign. She passed a bill out of her committee this week that would bypass President Obama and approve the Keystone XL pipeline. The question is whether the emphasis on energy will be enough to overcome a capable Republican opponent in Rep. Bill Cassidy in a state where the president is deeply unpopular. Those are big obstacles to overcome. (Previous ranking: 4)

3. Montana (D): Both appointed Sen. John Walsh (D) and Rep. Steve Daines (R) easily dispatched nominal primary challenges on June 3 and formally began a race that both campaigns had already been waging for months. There's very little public polling in the race but the general consensus is that Daines starts the general election with an edge -- and is likely to benefit from a national political environment benefitting Republicans. (Previous ranking: 3)

2. West Virginia (D): Rep. Shelley Moore Capito (R) likely wrapped up this Senate seat in November 2012 when she abruptly announced for the seat even though Sen. Jay Rockefeller (D) had yet to announce his retirement. Democrats eventually convinced Secretary of State Natalie Tennant to run but she started the race at a distinct disadvantage because of Capito's early start and the dislike toward the national Democratic party in the state. A recent non-partisan poll put Capito up 49 percent to 38 percent. That seems about right. (Previous ranking: 2)

1. South Dakota (D): The Jackrabbit State remains our most likely seat to flip. But let’s make the case for this being in-play. Former governor Mike Rounds (R) remains a strong favorite against Democrat Rick Weiland, but this race also includes former three-term GOP (U.S.) senator Larry Pressler and former Republican state senator Gordon Howie (not to be confused with Gordie Howe) running as independents. Neither has raised any money, but maybe Pressler (and to a lesser extent, Howie) steal enough Rounds' votes that this is in-play. Weiland can hope. (Previous ranking: 1)

Chris Cillizza writes “The Fix,” a politics blog for the Washington Post. He also covers the White House.

Aaron Blake covers national politics and writes regularly for The Fix.

Sean Sullivan has covered national politics for The Washington Post since 2012.

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